The invention relates to polyolefin foam materials of high dimensional stability at elevated temperatures which are made from modified and unmodified polypropylenes.
The preparation of polyolefin foam materials from unmodified polypropylenes is known.
Aside from polypropylene homopolymers, propylene-ethylene copolymers (U.S. Pat. No. 5,527,573) are also used for the production of extrusion foams. The high proportion of open-celled structures and the required use of special release devices is a disadvantage in the use of standard polypropylene for the production of extrusion foams. The melt viscosity determines cell growth during the production of extrusion foams and therefore is a parameter of cell collapse. In the case of standard polypropylene, the viscosity decreases as the elongation load increases. As a result, cell growth is unstable and the cell size is not uniform. When standard polypropylene, carbon dioxide as a blowing agent and nucleating agents are used, the foam sheet must be taken off by special release rolls heated to 95.degree. C. (Dey, S., SPE-ANTEC 1996, 1955-1958).
The processing of unmodified polypropylenes and of unmodified polypropylene copolymers into particle foams by the suspension impregnation method requires expensive decentralized autoclaving equipment as operating sites. The foam is produced by impregnating polypropylene particles having diameters of 0.5 to 5 mm in an aqueous suspension with 0.1 to 0.4 kg of blowing agent per kg of polymer at a temperature of 5.degree. to 20.degree. C. below the crystallite melting point of the polymer and subsequent expansion by foaming (European Patent 0 453 836, German patent 40 12 239).
To achieve a uniform cell structure for the suspension-impregnation method using unmodified polypropylenes, expensive technological measures are required, such as flushing the reactor content with gas at the reactor bottom during the discharging process (below the stirrer or through the stirrer) (European patent 0 630 935) or adding a heating fluid during the emptying of the autoclave, until 3/4 of the particle foam has been discharged (European patent 0 280 943). High shrinkage is a further problem of particle foams, which have been produced from unmodified polypropylenes using butane as a blowing agent. The shrinkage can be reversed by an expensive treatment of the shrunken particles at a temperature less than 20.degree. C. below the softening temperature under the action of compressed air or a compressed inert gas (EP 0 687 709).
It is furthermore known that, by using polypropylene blends and modified polypropylenes, polypropylenes can be processed on normal polyethylene standard foam extruders into extrusion foams and extrusion particle foams.
Known blend components for polypropylene for the production of extrusion foams on normal polyethylene standard foam extruders are propylene-ethylene copolymers (European patent 0 291 764), polyethylene (British patent 2,099,434), EVA (British patent 2,099,434), butyl rubber (European patent 0 570 221), styrene butadiene rubber (U.S. Pat. No. 4,467,052) and methacrylate copolymers (U.S. Pat. No. 5,508,307). These blending components, however, lead to a large decrease in the dimensional stability at elevated temperatures, the stiffness and the compression strength of the polypropylene extrusion foams.
Known modified polypropylenes, which are suitable for the production of extrusion foams and extrusion particle foams, are silane-grafted polypropylene (European patent 0 646 622) and polypropylene modified by polymerizable peroxides (WO 94 05 707) and polypropylene modified by high-energy electron radiation (European patent 0 190 889, De Nicola, A., Polymeric Materials Science and Engineering (1995), 106-108). The use of these modified polypropylenes in mixtures with unmodified polypropylenes is also possible.
The very high costs of the safety measures for the biological shielding of the high-energy radiation are a disadvantage of the method of modifying polypropylene by high-energy electron radiation. These safety measures make it necessary to shield the irradiation chamber hermetically by means of special concrete walls or lead segment constructions, to employ an expensive measuring technique as radiation protection and to monitor the operating personnel medically.
It is an object of the present invention to develop polyolefin foam materials of high dimensional stability at elevated temperatures from modified polypropylenes and unmodified polypropylenes, which avoid the known disadvantages of known products. It is a further object of the invention that the modified polypropylenes, used for the production of the polyolefin foam material, be readily accessible by a technologically simple method.